Our work is designed to provide fundamental understanding of RASopathies, a large class of developmental abnormalities caused by the germline mutations in components of the RAS/MAPK pathway. Patients with these conditions display a broad spectrum of phenotypes, including heart defects, short stature, facial dysmorphisms, and neurocognitive delays. Whole genome sequencing of RASopathies provides new sequence variants in the well characterized components of the RAS pathway, but the functional consequences of these sequence variations is poorly understood. We will quantitatively characterize the functional and phenotypic consequences of the activating mutations in MEK, a core component of the RAS pathway. Focusing on the same group of mutations, we will first determine their effects on the regulation and enzymatic activity of MEK (Aim 1). This will be done using mass spectrometry-based kinetic assays with purified proteins. Next, we will quantify the consequences of activating mutations for the kinetics of RAS signaling in vivo (Aim 2). This will be done using a quantitative imaging approach, with which we will monitor RAS signaling in Drosophila embryos. Finally, we will determine how the same mutations influence RAS-dependent tissue morphogenesis (Aim 3). This will be done using live imaging experiments in zebrafish, focusing on heart development as a model of a morphogenetic event that is commonly affected in RASopathies. Our studies should provide mechanistic insights into the biochemical and morphogenetic effects of mutations identified in a large group of human developmental abnormalities and highlight the importance of using multiple models and quantitative approaches for answering a specific biological question.